private void negotiateBegin() throws IOException {
    walk.resetRetain(REACHABLE, ADVERTISED);
    walk.markStart(reachableCommits);
    walk.sort(RevSort.COMMIT_TIME_DESC);
    walk.setRevFilter(
        new RevFilter() {
          @Override
          public RevFilter clone() {
            return this;
          }

          @Override
          public boolean include(final RevWalk walker, final RevCommit c) {
            final boolean remoteKnowsIsCommon = c.has(COMMON);
            if (c.has(ADVERTISED)) {
              // Remote advertised this, and we have it, hence common.
              // Whether or not the remote knows that fact is tested
              // before we added the flag. If the remote doesn't know
              // we have to still send them this object.
              //
              c.add(COMMON);
            }
            return !remoteKnowsIsCommon;
          }
        });
  }
  private void markReachable(final Set<ObjectId> have, final int maxTime) throws IOException {
    for (final Ref r : local.getAllRefs().values()) {
      try {
        final RevCommit o = walk.parseCommit(r.getObjectId());
        o.add(REACHABLE);
        reachableCommits.add(o);
      } catch (IOException readError) {
        // If we cannot read the value of the ref skip it.
      }
    }

    for (final ObjectId id : have) {
      try {
        final RevCommit o = walk.parseCommit(id);
        o.add(REACHABLE);
        reachableCommits.add(o);
      } catch (IOException readError) {
        // If we cannot read the value of the ref skip it.
      }
    }

    if (maxTime > 0) {
      // Mark reachable commits until we reach maxTime. These may
      // wind up later matching up against things we want and we
      // can avoid asking for something we already happen to have.
      //
      final Date maxWhen = new Date(maxTime * 1000L);
      walk.sort(RevSort.COMMIT_TIME_DESC);
      walk.markStart(reachableCommits);
      walk.setRevFilter(CommitTimeRevFilter.after(maxWhen));
      for (; ; ) {
        final RevCommit c = walk.next();
        if (c == null) break;
        if (c.has(ADVERTISED) && !c.has(COMMON)) {
          // This is actually going to be a common commit, but
          // our peer doesn't know that fact yet.
          //
          c.add(COMMON);
          c.carry(COMMON);
          reachableCommits.add(c);
        }
      }
    }
  }